Field of the Invention
Embodiments of the present invention relates to the art of a control system for a vehicle having a continuously variable transmission, and especially to a vehicle control system configured to control speed ratio and driving force during downshifting of the continuously variable transmission.
Discussion of the Related Art
JP-A-2008-202678 describes a speed limiting device for a vehicle configured to limit vehicle speed upon exceedance of predetermined limit speed. The speed limiting device taught by JP-A-2008-202678 is provided with a kick-down switch for determining a fact that an accelerator pedal is depressed to a kick-down position. According to the teachings of JP-A-2008-202678, when kick-down is determined by the kick-down switch in speed limiting operation and a required throttle opening is larger than an actual throttle opening, the speed limit operation is cancelled, and shifting to a gear stage several stages lower from a present gear stage is inhibited. According to the teachings of JP-A-2008-202678, therefore, an abrupt downshifting can be prevented unless the required throttle opening is smaller than an actual throttle opening even if a kick-down is executed.
JP-A-2005-170280 describes a controller for a vehicle having a continuously variable transmission that is configured to prevent a temporal drop in a driving force resulting from an excessive speed change rate during accelerating the vehicle by a kick-down. To this end, the controller taught by JP-A-2005-170280 is configured to calculate a requested gear shift speed to change a gear ratio to achieve a target driving force. If the calculated requested gear shift speed is faster than a predetermined limit speed during a kick-down, the gear shift speed is limited to the limit speed and a fuel injection amount to an engine is set based on the requested gear shift speed and the limited gear shift speed. That is, according to the teachings of JP-A-2005-170280, the gear shift speed is limited to the limit speed during kick-down to moderate a downshifting operation while increasing the fuel injection amount to the engine to increase an engine power. For this reason, an unintentional temporal drop in a driving force can be prevented.
JP-A-2013-68272 describes a control apparatus for a vehicle having an automated manual transmission configured to increase a drive torque while suppressing a wheel slip when kick-down is performed. According to the teachings of JP-A-2013-68272, a shift stage (required shift stage) to be selected among a plurality of shift stages is decided on the basis of a traveling condition of the vehicle, and a shift stage of a transmission is controlled to the current required shift stage. If the required shift stage is moved to the low speed side by two stages or more while a wheel slip is not detected, a skip shift is carried out to shift the stage of the transmission from “a required shift stage before change” to “a required shift stage after change”. By contrast, if the required shift stage is moved to the low speed side by two stages or more while the wheel slip is detected, the skip shift is not performed, and the shift stage of the transmission is sequentially changed more than once by one stage from “a required shift stage before change” to “a required shift stage after change”.
In the conventional vehicle having a continuously variable transmission, a downshifting and an upshifting are carried out by varying a speed ratio gradually and continuously. If a kick-down described in the above-mentioned prior art documents is performed in this kind of vehicle, a speed ratio is changed significantly in such a manner to increase a driving force. In this situation, however, a power of a prime mover is consumed to increase a rotational speed of an inertial member of the engine or the continuously variable transmission. That is, a power loss occurs inevitably during execution of the kick-down, and such power loss is increased with an increase in a variation width of the speed ratio. In addition it takes longer time to achieve a desired driving force or acceleration if such power loss occurs. Thus, if the kick-down is performed in the vehicle having the continuously variable transmission, long time is required to achieve the desired acceleration or driving force and hence response and acceleration feel are worsened.